The present invention, in some embodiments thereof, relates to magnetic resonance and, more particularly, but not exclusively, to an ex-situ magnetic resonance probe.
Nuclear Magnetic Resonance (NMR) is a quantum mechanical phenomenon in which a system of spins placed in a static magnetic field resonantly absorbs energy when applied with a certain electromagnetic frequency. This phenomenon is exploited in many applications, such as spectroscopy and Magnetic Resonance Imaging (MRI), for obtaining information regarding the chemical and physical microscopic properties of materials.
A nucleus can experience NMR only if its nuclear spin I does not vanish, i.e., the nucleus has at least one unpaired nucleon. Examples of non-zero spin nuclei frequently used in MRI include 1H (I=½), 2H (I=1), 23Na (I= 3/2), etc. When placed in a magnetic field, a nucleus having a spin I is allowed to be in a discrete set of energy levels, the number of which is determined by I, and the separation of which is determined by the gyromagnetic ratio of the nucleus and by the magnetic field. Under the influence of a small perturbation, manifested as a radiofrequency magnetic field (commonly referred to as B1, which rotates about the direction of a primary static magnetic field (commonly referred to as B0), the nucleus has a time dependent probability to experience a transition from one energy level to another. With a specific frequency of the rotating magnetic field, the transition probability may reach the value of unity. Hence at certain times, a transition is forced on the nucleus, even though the rotating magnetic field may be of small magnitude relative to the primary magnetic field. For an ensemble of spin nuclei the transitions are realized through a change in the overall magnetization.
Once a change in the magnetization occurs, a system of spins tends to restore its magnetization longitudinal equilibrium value, by the thermodynamic principle of minimal energy. The time constant which control the elapsed time for the system to return to the equilibrium value is called “spin-lattice relaxation time” or “longitudinal relaxation time” and is denoted T1. An additional time constant, T2 (≦T1), called “spin-spin relaxation time” or “transverse relaxation time”, controls the elapsed time in which the transverse magnetization diminishes, by the principle of maximal entropy. However, inter-molecule interactions and local variations in the value of the static magnetic field, may alter the “intrinsic” value of T2, to an actual observed value denoted T2*.
In conventional NMR spectroscopy or MRI systems, the sample to be investigated is placed in the bore of a static magnet. Whilst the conventional approach is generally preferable in terms of cost effective generation of the strong and uniform static magnetic field required for NMR measurements, the particular circumstances of some applications demand measurements which can only be achieved with a remotely-positioned instrument (commonly termed “ex-situ” NMR).
In recent years ex-situ NMR has become an increasingly important measurement technique in many applications, particularly oil well logging, and material research applications. Ex-situ NMR is different from conventional NMR spectroscopy and imaging insofar as the investigated sample is outside the apparatus. Therefore the static and rotating fields are typically far from being homogeneous. From the point of view of the measurement the sample may be infinite in size but the volume which contributes useful signal is limited.
Ex-situ NMR is also referred to in the literature as: “inside-out NMR”, “external field NMR”, “remotely positioned MR”, “projected field MR” and “one-sided MR”.
U.S. Pat. No. 7,358,734 discloses a sensor for ex situ magnetic resonance profiling with microscopic resolution. The sensor includes a magnet system with two pairs of permanent magnet blocks which are oppositely polarized for producing a magnetic field constant in a plane external to the body. A radiofrequency circuit is placed between the pairs.
Perlo et al. [Science, Vol. 315, No. 5815, pp. 1110-1112 (published online Jan. 10, 2007)] disclose a technique in which a variety of permanent magnet blocks, as well as “shim coils” are employed to homogenize the magnetic field just outside the probe.
Another technique, [Perlo et al., Journal of Magnetic Resonance 180 (2006) 274; Perlo et al., Science, 308 (2005) 1279] uses a similar portable nuclear magnetic resonance sensor with a single-sided open probe design. The probe includes a U-shaped main magnet, an inner magnet and a rectangular surface radiofrequency coil. The dimensions of the coil and the position of the inner magnet are adjusted to optimize the correspondence between the static and radiofrequency magnetic fields so that their spatial dependence has the same functional description (up to a constant). The resulting magnetic field inhomogeneity is compensated by a special pulse sequence that takes advantage this identical functional and results in an NMR signal called “nutation echo”. This probe can acquire fluorine-19 spectra of liquid fluorocarbons with 8 parts per million (ppm) resolution.
Additional background art includes Meriles et al., Science 293 No. 5527, 82-85 (2001); Eidmann et al., Journal of Magnetic Resonance A 122 (1996) 104; and Blank et al., Magnetic Resonance in Medicine, 54 (2005) 105.